The present invention relates to a device for detecting insipient tears on the edges of a sheet during manufacture, such as for example a sheet of paper, cardboard or the like.
The manufacture of a sheet of paper is carried out on a papermachine permitting successively the shaping, drying and rolling of the sheet. Throughout the manufacture, the said sheet is subjected to a large number of stresses due either to shrinkages during drying, or to variations in the moving speed of the various cylindrical members provided for driving and drying the sheet, the speed of which members is difficult to control. Also, the sheet of paper very often undergoes a surface wetting treatment which temporarily makes it delicate, and even more so as the manufacturing speeds are generally around several hundreds of meters per minute.
Under these conditions, it is easy to understand that this material of which the thickness is but a few tens of microns and which is relatively fragile, can suffer tears and even cracks which interfere with the manufacturing program and in the end prove very expensive.
It is therefore important to be able to detect as accurately as possible any incipient tears occurring on the edge of a sheet during manufacture and which are the forerunning sign of a forthcoming tear.
Very simple devices are already known, these devices being constituted of a light source, generally a filament lamp, and of a cell placed on the other side of the sheet. The sheet of paper then moves between the light source and the cell which, at that moment picks up the light transmitted through the sheet. As long as the sheet of paper passes normally between the light source and the cell, the transmitted light intensity is low. On the contrary, when there is an insipient tear, the cell receives all the light intensity emitted by the source, this triggering alarm means.
Although this device is widely used, it has however many drawbacks. First of all, the light emitted by the source spreads in all directions. As a result, the quantity of light received in one point of the sheet is very small, which affects the detection. Also, the detection cells are affected by the surrounding light radiations, since their receiving angle is too wide. Finally, with time, the detection device becomes dirty, since fibers and filling materials deposit on the optical surfaces provided for allowing the light beam to pass through, and then the device becomes unusable.
In order to obtain the best possible conditions of detection, it would then be necessary for the quantity of light received by the cell to be very small when the sheet of paper passes between the source and the cell, and very large when the light emitted by the source reaches directly the cell, which has not been possible heretofore.